1. Field of Invention
The present invention relates to light guides, particularly to light guides for use in flat panel displays, and more particularly to light guides for use in liquid crystal display panels.
2. Description of Related Art
In a liquid crystal display (LCD) panel, the liquid crystal (LC) layer does not emit light inherently, as compared to, for example, an electro-luminescent material layer in a plasma display panel. The LC layer or panel functions as a modulator of light transmitted through the LC layer. In a backlit LCD panel, a backlight module serves as a planar light source, which is an important element of the LCD panel, and is critical for enhancing the brightness of the LCD panel. The backlight module includes a light guide plate, for guiding light from a planar array of light sources (e.g., a planar array of LEDs), or a linear light source (e.g., a cold cathode fluorescence (CCF) tube, or a row of LEDs). Backlight modules having the latter linear light source are referred in the art as edge-lid backlight modules.
Heretofore, conventional light guide plates incorporated in edge-lit backlight modules utilize total internal reflection to direct light from a light source positioned at an edge surface of the light guide plate to a light emitting major planar surface facing towards the LC panel. The light from the light source is guided to propagate through the light guide plate to an opposite edge surface of the light guide plate, so that light can be emitted from the entire planar surface towards the LC panel. In the past, the other major planar surface of the light guide plate (e.g., the bottom surface) opposite to the light emitting planar surface is provided with a structure that facilitates reflection of light towards the top planar surface. For example, in some prior art light guide plates, a reflective layer may be provided at the bottom surface, reflecting light back into the light guide plate. In other prior art light guide plates, the bottom surface of the light guide plate opposite to the light-exit surface is textured with light-reflecting or light-scattering surface dots. Light beams encountering these dots are diffused and/or reflected towards the light emitting surface of the light guide. More particularly, the light-reflecting or light-scattering dots formed or embedded on the bottom surface of the abovementioned conventional light guide plate are capable of changing the direction of propagation of light beams so as to spread light from the linear light source across the planar light emitting surface, thereby forming a planar light source emitting light from a plane. The diffused and reflecting surface of the light-reflecting or light-scattering dots are generally matte as a result of the fabrication process. Accordingly, a great deal of energy is required by the point or linear light source to produce planar light output of acceptable brightness, resulting in waste of electrical power consumption.
Another technique using a V-cut light guide plate has been developed in the field of backlight modules. The V-cut light guide plate is made mainly by directly micro-fabricating prisms on a light guide plate, and is usually used in a backlight module in combination with a reverse prism sheet. As shown in FIG. 1, the prior art backlight module mainly includes a reverse prism sheet 3, a V-cut light guide plate 2 having a tapering thickness, and a reflective film 1 under the reverse prism sheet, and a diffusion film 4 above the reverse prism sheet. The LC panel (not shown) is placed on top of the diffusion film 4. A linear light source/reflector 5 is positioned at the thicker edge of the light guide plate 2. Compared to the earlier conventional backlight modules, the brightness of the backlight module having a V-cut light guide plate can be enhanced by nearly 30%. Therefore, overall power consumption for a particular output brightness can be saved by about one third using such improved backlight module, which is a considerable improvement on energy-saving performance.
Light guide plates used in backlight module are typically made by injection molding. In general, molten polymethyl methacrylate (PMMA) material is filled into a mold cavity having the desired mold structure and a holding pressure is maintained. After the material is allowed to cool down and harden to conform to the configuration of the mold cavity. However, when it comes to V-cut light guide plate, a high failure rate is present as a result of difficulties associated with making the mold cavity and the nature of the above-described injection molding process. The replica of the mold master having V-cut structures made by CNC precision-machining are often mal-manufactured and/or result in defects. Therefore, the manufacturing of V-cut light guide plate becomes more difficult and thereby less cost effective.
Furthermore, the problems with prior art light guide plates are exacerbated by the tightened requirements of lighter-weight, thinner-profile and more flexible structure for LCD devices, which are gradually becoming the mainstream in recent years. Due to the limitations of injection molding, it is not easy to obtain thin and flexible light guide plates manufactured by this method. In particular, when it comes to large LCD panels sizes, the failure rate is even higher.
There remains a need for a back light plate having a structure that is thin and flexible, and efficient in planar light output and easy to manufacture.